A method and an apparatus of the generic type are known from DE-OS 22 59 818. Different plastic materials are melted there in two or more plasticizing units, and are injected into an injection molding tool by a special, controllable multicomponent injection nozzle. This nozzle has a quite complicated structure, for the precise control of the individual melt streams of the respective plastic components.
Other melt injection devices for injection molding plastics are otherwise adequately known in the prior art. For example, from WO 98/09768, an injection molding apparatus is known whichxe2x80x94in contrast to the otherwise usual direct injection of plastic melt from the plasticizing and injection unitxe2x80x94introduces melt into a receiver chamber, and subsequently presses this melt into the mold cavity by means of a die. This reduces the operating pressure in the melt. The solution described there has a receiver chamber for receiving the plastic melt needed for one shot, the plastic being moved through a heated passage, by means of an infeed spindle, into a tube, and then, by means of a die, into the receiver chamber. Then the receiver unit is moved to a second position, in which the receiver chamber is flush with an opening into the mold. In this position, a second die then presses the plastic melt into the mold cavity.
As is also the case with this special, previously known solution, it is generally true that, during an injection molding process, the plastic material always must be introduced into the mold via a certain length of flow path. Due to the rheology and the particular fill technique, a high pressure must be applied at the injection point to inject the melt. The pressure then markedly decreases along the flow path. Due to this pressure loss, the pressure at the end of the flow path is much lower than exists at the injection point. This pressure loss along the flow path has the result that very high pressures are needed at the injection point, to assure that the pressure level necessary for completely filling the mold cavity will also exist at the end of the flow path.
To minimize the high pressure losses which occur along long flow paths, it is customary to multiply the gating of molded parts, i.e., the mold cavity is equipped with several injection points. However, this has the disadvantage that weld lines form at points where the various flow fronts meet, thus mechanically weakening the molded part. Another disadvantage is that the several injection points cause strongly differing pressure conditions at various points in the mold cavity.
As a remedy, methods are known whose object is to reduce the interior pressures in the mold. One such method is injection-compression molding, where one works with positive molds. For example, with an enlarged wall thickness, the desired plastic mass is injected and then the mold is collapsed through the closure mechanism of the injection molding machine. The wall thickness is reduced in virtue of the fact that a compression process takes place. It thus becomes possible to assure a uniform compressive pressure, even as far as the end of the flow path. With other methods, the injection process is initially begun with a small wall thickness; when the plastic material is injected into the mold cavity, the wall thickness of the molded part is then increased, until the desired molding compound has been injected completely. Then the closure force is applied to the mold, and the wall thickness is reduced to the desired extent.
All these previously known methods share the disadvantage that special mold techniques and/or processes are needed to assure the lowest possible and uniform pressures in the mold cavity. This requires a quite large expenditure on apparatus and process engineering. Furthermore, in some circumstances, restrictions must be accepted as regards the geometry of the molded part.
As a result, the invention includes a method and an associated apparatus for injection molding plastic parts comprising several plastic components, so as to avoid or at least reduce the previously known disadvantages. The advantages of the known injection-compression technique should therefore be utilized, without having to accept its disadvantages. Furthermore, attention has to be paid that existing injection molding machines can be appropriately retrofitted without great expense.
In general, in accordance with an aspect of the present invention, a method is provided for the multicomponent injection molding of plastic parts, in which at least two different plastic components are injected into an injection molding tool to form a plastic part. The method includes: a) introducing a first plastic component including a first plastic melt into a melt accumulation chamber, which is bounded by a piston-like end section; b) subsequently introducing a second plastic component including a second plastic melt into the melt accumulation chamber; and c) transferring the plastic melt accumulated in the melt accumulation chamber into the injection molding tool.
In one embodiment, when the plastic melt is transferred in accordance with the above step c), the piston-like end section of the melt accumulation chamber is moved in such a way that it forms part of the cavity wall of the injection molding tool.
One idea of the invention therefore focuses on the injectable melt components initially being accumulated in a first accumulation chamber. Then the injection material is transferred to a second accumulation chamber, which is bounded by a piston surface. When the melts subsequently are injected into the mold cavity, the piston surface is moved in such a way that finally it becomes a region of the actual mold cavity. The piston surface here forms a certain part of the surface (projected as seen in the direction of the piston movement), over which the injection pressure can distribute itself uniformly. This makes the pressure distribution in the mold cavity more homogeneous. The necessary injection pressures can be reduced, which avoids or at least reduces the disadvantages described in the introduction.
It can also be specified that, after step b) and before step c) above, more plastic component is introduced into the melt accumulation chamber. This creates a layering of the plastic materials 1-2-1.
In one embodiment, the piston-like end section of the melt accumulation chamber is situated at an end position before the plastic melt begins to be transferred into the injection molding tool.
Precise control and regulation is quite important with the inventive method; however, this can be done quite simply with injection molding equipment of the prior art. In this connection, the invention can specify that the plastic melt accumulated in the melt accumulation chamber is transferred into the injection molding tool in a manner that is controlled or regulated at least part of the time. The regulation or control, in one embodiment, is effected as a function of the melt pressure in the melt accumulation chamber. The regulation or control can be effected as a function of time, path, or pressure.
In one embodiment, after the plastic melt accumulated in the melt accumulation chamber has been transferred into the injection molding tool, the follow-up pressure is applied to the melt through the piston-like end section of the melt accumulation chamber.
According to other embodiments, the region of the piston-like end section of the melt accumulation chamber is temperature-controlled at least some of the time. This can involve heating as well as cooling.
The method can also be combined with special production processes. In particular, such that, when the plastic melt accumulated in the melt accumulation chamber is transferred into the injection molding tool, a film situated there is back-molded. Alternatively, it is possible that when the melt in the accumulation chamber is transferred into the injection molding tool, a textile material situated there is back-molded.
It is possible to utilize the compression technique; then, after the plastic melt accumulated in the melt accumulation chamber is transferred into the injection molding tool, a compression process is performed through the piston-like end section.
An apparatus for the multicomponent injection molding of plastic parts is provided which includes: at least two plasticizing units to plasticize two plastic materials to provide a plastic melt, a melt accumulation chamber where the plastic melt produced by the plasticizing units is accumulated, and an injection molding tool with a mold cavity.
In one embodiment, the melt accumulation chamber is bounded by a piston-like end section, which is shaped in such a way that it forms at least part of the cavity wall of the injection molding tool.
In accordance with one embodiment, means for temperature control are disposed in the region of the piston-like end section of the melt accumulation chamber.
Furthermore, for precise control and regulation, the invention can provide that valve means are disposed in the melt flow paths so as to disable or enable the throughflow of plastic melt.
In one embodiment, the piston-like end section of the melt accumulation chamber is designed as the end piece of a pre-plasticizing cylinder, in which a plasticizing reciprocating screw is disposed, the pre-plasticizing cylinder being able to move axially. Another embodiment is that the pre-plasticizing cylinder stands in connection with moving means, by means of which the pre-plasticizing cylinder can be moved relative to the injection molding tool.
The pressure in the cavity is reduced especially advantageously if the projected area of the piston-like end section, in its directional motion, makes up a fraction of at least 0.5%, in one embodiment at least 5%, of the projected surface of the total cavity wall in the direction of motion.